This invention relates to magnetic resonance (MR) techniques, and more particularly to two-dimensional (2-D) magnetic resonance spectroscopy (MRS) techniques.
Quantification of chemical concentration using one-dimensional (1-D) MR spectra involves integrating the entire spectrum of a chemical, including all its peaks, along the chemical shift axis to determine the area under the curve. This technique can generally be employed when interference with the spectrum of the chemical of interest by other chemicals is tolerably low. In many samples, including complex biological samples (e.g., the human brain), many chemicals contribute to the 1-D spectrum of the sample, making it difficult to identify and quantify a spectrum for a single chemical of interest. One dimensional MRS techniques have been developed to reduce this interference, such as the multiple quantum filtering spectral editing method, see, e.g., Warren et al., Journal of Chemical Physics, 73(5):2512, 1980. These 1-D techniques acquire averaged signals with a constant echo time (te) and, therefore, the frequency dependence, or J dependence, of the MR signals is not utilized. Consequently, a peak that has a chemical shift coordinate close to a peak of the chemical of interest will overlap and thereby interfere with the nearby peak. In multiple quantum filtering spectral editing, a small residual of the interfering peak can materially alter the peak value of the edited peak of the chemical of interest. Addressing this issue generally involves precise calibration of editing pulse parameters.
Two-dimensional MRS resolves resonance peaks along an additional axis, a frequency axis (J axis), providing more information to differentiate chemical peaks. A 2-D MR spectrum maps frequency in the vertical dimension (D2) against chemical shift, which is the horizontal axis (D1). Thus, in 2-D MRS, peaks having similar chemical shifts can be distinguished in the frequency dimension. Using 2-D MRS (e.g., localized J-resolved 2-D MRS) to analyze samples containing multiple chemicals, peaks that could not be differentiated using 1-D MRS can be resolved, see, e.g., Ke et al., Psychiatry Research Neuroimaging, 100: 169, 2000; Ryner et al., Magnetic Resonance Imaging, 13(6):853, 1995; Weber, Technology and Health Care 5:471, 1997, which are incorporated by reference.
MR signals from J-coupled protons are J modulated, or dependent on sin(xcfx80Jte) or cos(xcfx80Jte), where te is the echo time. Each echo corresponds to a free induction decay (FID) signal, which is recorded as data. In J-resolved 2-D MRS experiments, MR signals are acquired using enough different te values such that J modulations in signals can be assessed by performing a Fourier transform with respect to te, see, e.g., W. P. Aue et al., Journal of Chemical Physics, 64:4226, 1976; R. R. Ernst et al., Principles of Nuclear Magnetic Resonance in One and Two Dimensions, 1988, which are incorporated by reference. According to the relationship between the time and frequency domains in Fourier transform theory, the increments in te, xcex94te, determine the bandwidth of the J-frequency dimension. The bandwidth is equal to the inverse of the increments in te, 1/xcex94te. In 2-D MRS, xcex94te is selected to provide a bandwidth that covers the maximum J value of interest and can be adjusted to maximize frequency resolution and signal intensity. For most biological metabolites, J values range from 0 to 20 Hz. Therefore, to use 2-D MRS to assess metabolite concentrations in a region of the human brain, a suitable xcex94te can be, e.g., 10 milliseconds (corresponding a bandwidth of 100 Hz) and can be adjusted to produce different bandwidths.
The present invention concerns identifying appropriate peaks in 2-D MR spectra for 1-D extraction and comparing spectra for test samples to those obtained with reference samples to obtain quantitative measurements of chemical concentrations. These methods can be used to obtain information about, e.g., in vivo chemical concentrations in the human brain using 2-D MRS and offer advantages over 1-D MRS techniques. Test samples can be biological samples, e.g., human or animal tissue, or non-biological samples.
Identifying a suitable peak in a 2-D spectrum for analysis by 1-D extraction involves subjecting a reference sample to a MRS sequence, obtaining a 2-D MR spectrum of the reference sample, and selecting a peak for a chemical of interest in the 2-D MR spectrum by comparing the peaks in the 2-D MR spectrum. It is generally useful to identify peaks that are substantially distinct from other peaks in the 2-D spectrum to reduce interference from other resonance peaks. The peak that is identified can be the peak that is most distinct from any other peaks in the 2-D MR spectrum of the reference sample or the strongest peak that is substantially distinct from the other peaks in the 2-D MR spectrum. With a peak identified, a test sample comprising the chemical of interest is subjected to a MRS sequence, a 2-D MR spectrum of the test sample is obtained, and a 1-D spectrum, i.e., 1-D slice taken along the chemical shift axis, comprising the identified peak for the chemical of interest is extracted from the 2-D MR spectrum of the test sample.
This peak identification methodology can be applied to samples that contain multiple chemicals. If the identified peak for the chemical of interest overlaps with a peak associated with other chemicals in the 2-D MRS spectrum of the test sample, then the 2-D MRS for the reference sample comprising the chemical of interest can be reviewed again to identify the xe2x80x9cnext bestxe2x80x9d peak, i.e., another peak substantially distinct from other peaks in the reference sample. In addition, 1-D MR spectra can be extracted from the 2-D MR spectra of multiple test samples. These techniques can be used to observe chemical peaks in 2-D MR spectra that do not suffer from substantial interference with either other peaks from the same chemical or peaks from other chemicals in the test sample. The chemical of interest in these samples can be a metabolite, e.g. gamma-aminobutyric acid (GABA), creatine (Cre), N-acetyl aspartate (NAA), choline (Cho), glutamine, glutamate, alanine, taurine, myo-insitol, glucose, aspartate, or lactate. The identified peak for GABA can be at a chemical shift of about 2.94 ppm, and the identified peak for Cre can be at a chemical shift of about 3.08 ppm.
The invention also generally features methods for quantitative assessment of chemical concentrations. To obtain concentration data, two reference samples containing first and second chemicals, respectively, and a reference chemical (e.g., 3-(trimethylsilyl)-1-propane-sulfonic acid) are subjected to a MRS sequence to obtain 2-D MR spectra. One-dimensional MR spectra, each containing a peak for the first or second chemical, are extracted from these 2-D MR spectra. The selected peaks can be substantially distinct from any other peaks in the 2-D spectra.
Standardized measures of intensity are calculated for each reference sample using the extracted 1-D spectra. This measure of intensity (k) is a ratio of the area under the peak of the first or second chemical (A) in the extracted 1-D MR spectrum of the reference sample to the area under a peak of the reference chemical (Aref), multiplied by the concentration of the reference chemical (xcfx81ref), and divided by the concentration of the first or second chemical (xcfx81):
k=(A xcfx81ref)/(Arefxcfx81).
Using the measures of intensity for the first and second chemicals, a comparison ratio (R21) equal to the ratio of the standardized measure of intensity for the second chemical (k2) to the standardized measure of intensity for the first chemical (k1) is calculated:
R21=k2/k1.
This comparison ratio can provide information about the relative concentrations of the first and second chemicals in a test sample. One-dimensional extractions from 2-D MR spectra of a test sample are obtained by subjecting a test sample including the first and the second chemicals of interest to the MRS sequence, obtaining a 2-D MR spectrum of the test sample, and extracting 1-D MR spectra that respectively contain the identified peaks for the first and second chemicals. A test sample ratio (T21) is calculated by taking the ratio of the area under the peak of the second chemical in the extracted 1-D spectrum for the second chemical (Atest2) to the area under the peak of the first chemical (Atest1) in its extracted 1-D spectrum:
T21=Atest2/Atest1.
Dividing the test sample ratio (T21) by the comparison ratio (R21) yields a relative measure of the concentration of the second chemical (C2) with respect to the first chemical (C1) in the test sample:
C2/C1=T21/R21.
These quantification techniques can utilize the same parameters as the peak selection techniques and can also be performed repeatedly to obtain measures of relative concentrations for multiple chemicals. Different peaks from the 1-D extractions of the 2-D MR spectra of the reference samples can also be iteratively selected in an attempt to reduce overlap in the 1-D extractions. The calculations can be simplified by using the same concentrations of the reference chemical and/or first and second chemicals in the two reference samples (i.e., where xcfx81ref1=xcfx81ref2 and/or xcfx811=xcfx812). A Marquadt non-linear curve fitting algorithm is useful for calculating the areas under the peaks. These methods can also provide absolute measures of concentration if a specific concentration of a chemical in the test sample is known or can be estimated.
The 2-D MRS sequence used in these methods can be a J-resolved magnetic resonance spectroscopy sequence and can include a hard 180 radio frequency pulse between two data acquisition periods to reduce total scanning time. Another approach to reduce scanning time is to include interpolation for exponential decay to generate a greater number of data points than the number of data points acquired during MR scanning. Each of these approaches to reducing total scanning time can independently halve the time required.
These new methods offer several advantages compared to quantification techniques using 1-D MR spectra. Reductions in sensitivity to editing pulse parameters and contamination due to T2 decay can be achieved using 2-D MRS techniques. With 2-D MRS, all the available signal can be detected and used as input in the quantitative methodology. In addition, many chemicals can be quantified using 2-D spectra, including most protonated metabolites with J-coupling, e.g., GABA, glutamine, lactate, and alanine. 2-D MRS techniques also enable analysis of a wide range of samples, such as human and animal tissue. These techniques are especially well-suited to study most regions in the human brain.
Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, suitable methods and materials are described below. All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety. In case of conflict, the present specification, including definitions, will control. In addition, the materials, methods, and examples are illustrative only and not intended to be limiting.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.